A challenge in electroplating uniform metal layers in manufacturing semiconductor and other micro-scale devices is producing and maintaining a desired electrical field at the surface of the wafer or substrate. The distribution of electrical current in the plating solution is a function of the uniformity of the seed layer across the contact surface, the resistance of the seed layer, the configuration/condition of the anode, electrolyte flow characteristics and the configuration of the chamber. However, the current density profile on the plating surface can change. For example, the current density profile typically changes during a plating cycle because as metal is plated onto the seed layer, its electrical characteristics change. This can occur within a few seconds, or even in a fraction of a second. Current density can change over a longer period of time because the shape of consumable anodes changes as they erode and the concentration of constituents in the plating solution can change. Therefore, it can be difficult to maintain a desired current density at the surface of the wafer, to form uniform void-free plated layers, and to achieve a final profile shape of the plated layer as desired for subsequent processing.
As one particular example, the current density can be significantly higher near the edge of the wafer and at the junctions between the contact elements and the wafer than at other locations on the wafer. This is referred to as the “terminal effect.” The terminal effect can result in electroplated layers that are not uniformly thick, contain voids, or have impurities or defects. These tend to reduce the manufacturing yield of defect-free devices.
Electroplating systems currently used in the semiconductor industry have two or more electrodes, typically set up as anode plates or rings. The distribution of electrical current provided by each anode may be actively varied the during the plating process. This dynamic current control may be used to achieve a specific profile for a conductive layer on the workpiece or to account for temporally and/or spatially varying characteristics of the electrical processing.
Some electroplating systems also have an optimization capability that can select and adjust electrical processing parameters, specifically the current profile over time for each anode. The optimization adjusts the electrical processing parameters in accordance with either a mathematical model of the processing chamber or experimental data derived from operating the actual processing chamber. In these optimization systems, after a workpiece is processed with the initial parameters, the actual results on the plated substrate are measured. A sensitivity matrix based upon the mathematical model of the processing chamber is then used to select new parameters that correct for any deficiencies measured in the processing of the first workpiece.
These parameters are then used in processing a second workpiece, which may be similarly measured, and the results used to further refine the processing parameters. These optimization systems can also profile the seed layer on a substrate before the electroplating process. This information can then be used to determine an initial set of process parameters designed to electroplate a metal layer onto the seed layer in is way that compensates for deficiencies in the seed layer.
The seed layer is generally formed on the substrate using chemical vapor deposition (CVD), physical vapor deposition (PVD), electroless plating processes, or other suitable methods. The seed layer is needed for subsequent electroplating because electroplating requires a conductive surface, and the bare substrate, such as a silicon wafer, is generally not sufficiently conductive. The trend in semiconductor technology is towards thin seed layers required for producing smaller features. Thin seed layers are also faster to apply and may have other advantages as well. On the other hand, electroplating onto a thin seed layer further increases the engineering challenges of designing and controlling electrochemical plating systems. Very thin seed layers, for example having a sheet resistance of 50 Ohms/sq or higher, require more extreme chamber electrode current adjustments, and make it more difficult to predict how the sheet resistance changes during the plating process.
Accordingly, there is a need for electrochemical plating systems and methods better adapted to processing substrates having thin seed layers.